Impressive work. I'll need to study it a bit longer to absorb more of it, but don't doubt what I take to be your central thesis which is the effect on timing and velocities of inertia, given the sheer volumes involved moving through the narrow opening.

Out of curiosity, I had a quick look at the tidal streams graph for the Rip in the Australian Hydrographic Office's Austides app (v2), excerpts copied below.

The profile of the rates chimes pretty much exactly with your No 4 graph. That is, there's no flat spot at all at the cross over from ebb to flood.


The next thing I looked at was the timing of the change from ebb to flood (I don't want to call it slack water because there is no "slack" period, which is one of your points). As you propose, the change over should be significantly behind the change in the external tide, as the inertia keeps water moving well past the top of, and the bottom of, the external tides (ie tidal levels outside the heads.)

So picking today, the high tide outside the heads at Lorne are at 01:25 and 13:14 and the low tides are 07:23 and 19:04. In Austides the cross overs from flood to ebb in the Rip are given as 05:02 and 15:58 and from ebb to flood 11:10 and 22:53. And as you can see below the first maximum rate of flood in the Rip is at 01:02, just 25 minutes before the rising tide has turned at Lorne, and the maximum rate of ebb at 4.4 knots in the Rip is at 07:38, 15 minutes after the tide has already turned to come back in, in Lorne.
If I haven't confused myself (entirely possible), then Austides seems to line up with what you've said.

hi Quixotic,

Thanks for your reply. Yes the Aus-Tides 2024 app current predictions for 4/11/2024 do show the steep speed changes into and out of slack water - no "flat spot", (sketch #3) style profile in sight!

I think that nearly all entrances have a similar behaviour but it is most important at PPH because the rate of change in speed is a lot higher than for most other places. I think it is because we can't "eyeball estimate" low current speeds at all well, that many folk (me included) think that at times it feels like a sketch #3 scenario, even though the science, instruments, (and safety) say otherwise.

I see that because Aus-Tides does not let you look at an "equal levels" time, you have tried instead to look at the inertia delay between "max outside" and "max current", both of which Aus-Tides can supply. While this can be attempted, there are several traps:

a) We really need the "max drive" time which isn't the same as "max outside". The "drive force" is the level difference (outside height - inside height). Now when the outside hits max height (and is briefly not changing), the flood current is screaming into the Bay with the inside level rising fast. So the height difference (or drive force) is already shrinking at that time.

The real point of "max drive" is a time somewhat before outside max, where the outside tide's rate of rise has slowed enough to exactly match the inside tide's rate of rise. My tide curves say that for your examples this occurs before the Lorne Hi/Lo times by 24, 24, 12, & 30 minutes respectively.

b) At max flood or ebb flow much of the flow resistance is concentrated around the Heads and is also very sensitive to tide height wrt MSL as this determines how much the large fringing reefs are exposed, or how deep they are submerged. Also the current curves don't have nice sinusoidal tops & bottoms so picking off peak flood or ebb times is more error prone.

In the gentler middle two cases in your table, my system gave inertial delays of 42 & 48 mins between "max drive" and "max rate". However in the outer two higher speed cases the current waveforms get quite distorted and I see delays of only a few minutes. Not sure how to interpret that. Note that when approaching slack water of either type, the submerged depths of reefs between the headlands are similar and not highly variable as in both of the max flow cases.

One more comment before moving on is that for places like Lorne sitting directly on the coast we might use the terms "in" and "up" interchangeably for tides. However near PPH, and many other places, the language needs to be a bit more precise because all four combinations: "in & up", "in & down", "out & down", and "out & up" can occur when talking about streams & heights.

I have appended my DIY modelling results (in blue) to your 4th November table. Included is the extra info on "Equal Levels" (time & speed) and also the "Overshoot" (time to slack & reverse level difference at slack).


The red lines show which columns can be compared (I didn't organise the layout very well). Aus-Tides max current speeds are for the channel centreline which due to a strong "ebb jet" effect are somewhat stronger than my numbers derived from volume outflow rate rather than centreline speed.

The agreement isn't bad and I suspect the truth might be somewhere in between. In reality the difference is not important in view of weather uncertainties. ( Graeme I'm still working on a reply - I have a plot somewhere? of the June 2014 "big daddy" storm which shows the strong weather effects possible. )

Jake

Thanks. You've a much more sophisticated understanding of this than I expect I'll ever have.

I haven't navigated the Rip, tho' I hope to someday. But it certainly has a reputation that makes one want to be prepared.

In general terms, I suppose the information one wants is what strength/direction of current, when. If the Austides app tidal stream graph is correct, then from that one can read off expected rates at various points in time. Today (9 Nov), were I entering Port Phillip through the heads, from 2NM, 200 degrees True (ie a bit west of south) off Pt Lonsdale, I might look to be heading in at 07:00 when the inflow rate is 2.05 knots. An hour later it would be .06 knots outgoing. By then I should have made it 6 NM (conservatively) through the heads and if turning to follow the South Channel I'd be north of Portsea. Still in your choke area, but likely to make deeper water via the Pinnacle channel in the next hour, by which time the stream at the Rip would be 2.51 knots outgoing, but milder than that further into the Bay. Perhaps I should look to start in at 06:30 instead of 0700, to take more advantage of the incoming stream (2.7 knots incoming at 06:30) to get through the heads and up the channel a little quicker and earlier.

Someone who's done it may want to comment on whether that's a sensible approach.

If the above is OK on the face of it, a key question is whether there are other things which have to be taken into consideration which may cause the streams to depart significantly from Austide predictions. 'Significantly' of course varies, if I'm in my 6 knot cruising monohull, then a knot or two in rate, or an hour or more of time difference would be good to know. If I am in a 12 knot catamaran, my tolerances would be higher. And if I am in a 20+ knot power boat, then I may care less.

You mention winds, barometric pressure differences and storm surges.

It would be great if these could be simplified into 'rules of thumb' which can be used in practice. So if there's lower barometric pressure over Bass Strait versus PPB or vice versa, how many millibars difference has roughly what order of effect on timing and rates of streams? If there is a Northerly that has been blowing for several hours, what strength and duration of wind will affect timing and rates? Ditto a Southerly. From the work you've done, referring to the Pt Richards tidal gauge readings seems like the best way to get a handle on storm surge. But if any of these generally have effects that are less than 30 minutes in time and no more than a knot or so in speed, then one can likely live with them, without having to adjust one's timing. Knowing, (if possible) the thresholds when they each or together may exceed 30 mins and 1 knot, would provide a rule of thumb for when to assess them more exactly and adjust one's timing.

Those with experience of the Rip may wish to comment.

Hi Guitz,

Thanks for adding your experience to this thread.

Glad it shows you expect slack a little before Hi/Lo at Williamstown. Many folk take slack after flood as Williamstown Hi tide time with the reason: "Cos that's when the Bay is full". This doesn't take into account that the waters near the Heads have been falling for nearly 3 hours, so the total water volume would be falling by then, meaning the Ebb at the Heads had begun.

So the true max volume time (& slack) should be a little earlier than Hi tide at Williamstown, so the still slightly rising level there exactly offsets the falling water volume near the Heads. Theoretical calculations show this offset time would be around 17 minutes before Hi tide at Williamstown if it's a big tide. Your rule of thumb is suggesting 25 minutes. Better earlier than late!

IMHO folks who use the Williamstown Hi/Lo times might be better off using West Channel Pile times or Even Hovell Pile Hi/Lo times. My GPS tracker runs often show Heads stream reversals between the Hi or Lo times of these two stations.

You said:
A year or so ago i watched a fishing youtube on the rip area and the idiot presenter gave his viewers the impression slack water was at high tide. That could get someone killed. I sent him an email and posted in the comments section. Stupid idiot didn't want to admit he got it wrong !

Sadly a fair bit of that goes on. Such bad juju can indeed get people killed. That's what motivated me to slip in POST #2.

The real picture is that slack after flood is always associated with high tide INSIDE a Bay, which depending on the degree of constriction at the entrance, may be from ~0 to 3.5 hours AFTER high tide at the entrance.

All interesting and good stuff Guitz.
The one bad part is your reference that particular "divers perspective article". <-- In places it stinks.

Unfortunately there is dangerous info in there about very delayed slacks - a myth which interestingly is dangerous advice to his very own customers. I fought it for many years, and as you say: didn't want to admit he got it wrong !

Has a new owner now, so I'll try my luck again soon. Hopefully it will eventually be made safe.

Jake